Process and quality control of semi-periodic microstructures with structures ranging from microns to nanometers plays an important role in fabrication of optoelectronics, optical and microelectronic devices. One approach to perform quality control on a microstructure is to detect the diffracted light intensities from the microstructure. The light intensities for the different diffracted orders together with a computer algorithm may then be used to produce a topographic image of the microstructure.
Process and quality control of microstructures includes illuminating the microstructure with broad banded light, and measuring the diffracted intensities for all wavelengths and diffracted orders. These data are then used to generate a topography image of the microstructure using a computer algorithm. The illuminating optics is positioned in the vicinity of the microstructure surface and perpendicular to it for optimal detection of the diffracted intensities. The diffracted light orders are collected by collimating optics and incident on a detector system. The detector system may collect those orders parallel or one at a time.
For each wavelength the intensity of the diffracted orders form a distributional pattern, which may be used to uniquely determine the microstructure topography using a computer algorithm. The measurement is rapid and non-destructive since the invention is based on an optical technique.
There are a number of existing techniques available for measuring the surface topography of samples having such small structures. These include:
Electron beam imaging is a technique where a focused electron beam is provided to create an image of a specimen. For microstructure studies the prevalent method of using a focused electron beam is the scanning electron microscopy (SEM). The electron beam creates secondary electrons originating from the upper part of the surface and the generation of these electrons depends on material and geometry. Thus an image is obtained by measuring the current of secondary electrons from scans of the beam across the surface. SEM is capable of measuring features of 1 nm.
However, electron beam imaging has some drawbacks:    1. The method does not reveal depth information    2. Destructive method when profile of the structure is measured by cross-sectional technique    3. Sample environment during measurement is vacuum    4. Conductive coating of non-conductive structures is necessary in order to enhance the generation of secondary electrons. This alters the structure to be measured.    5. Build-up of surface charge affects the measurement resulting in distorted images
Scanning probe microscopy (SPM) utilizes a small mechanical probe brought very close to the surface of the specimen detecting the proximity of surface atoms. By this technique an image of the surface topography is obtained by scanning the probe across the surface recording the vertical (height) adjustments of the probe such that a constant response between surface and probe is maintained. The atomic force microscope (AFM) belongs to the group of SPM's that sense the atomic forces between the probe and surface. The technique gives features of few nanometers, however the technique suffers from    1. Topographical distortions of image due to folding of probe shape and surface.    2. Structures having high aspect ratio cannot be measured.
Both the electron beam imaging and the scanning probe microscopy suffer from being operator demanding and time consuming. Another limitation to these techniques is lack of measuring embedded structures in samples.
Optical profilometry is a method where a light beam having a small spot size is applied to produce an image of the surface of a sample. Interference between light reflected from the sample and a known surface is used to obtain height information. Thus, scanning across the surface provides the topographic image of the sample. The method has potential of conducting vertical measurements very accurately. However, resolution of lateral features is limited by the spot size of the light beam, which is in the range of a micron.
In contrast, the herein presented method and apparatus of the invention provide with an improved characterization technique to obtain spatial features of symmetric and asymmetric semi-periodic structure located at the surface or embedded in nearly planar samples. The apparatus collects sufficient data for uniquely determination of the arbitrary shaped profile. The measurement of the semi-repeated structure is non-destructive, fast, accurate, reproducible and reliable.
U.S. Pat. No. 5,963,329 discloses an apparatus and a method for determining the profile of periodic lines on a substrate. The substrate is illuminated with a broadband light and light diffracted by the structure is collected, measured and recorded as function of wavelength. The determination of the profile is conducted as follows. The intensity of the diffracted light is calculated from a seed model of the profile using Maxwell's equations. Comparison between the calculated intensity and the collected intensity vs. wavelength curve is performed to adjust the parameters of the model such that the measured and modeled intensities eventually match.
However, the apparatus and method of U.S. Pat. No. 5,963,329 has the following disadvantages:    1. It may only be applied to symmetric periodic line structures,    2. the recorded data used in the profile determination is only sufficient for symmetric structures. An attempt to extend the described invention to asymmetric profiles will lead to un-determined equation systems that are incapable of calculating the profile in a unique way, and    3. it may only be applied to repeated structures having collinear grating vectors.
U.S. Pat. No. 6,281,974 discloses a method for measuring at least one desired parameter of a patterned structure having a plurality of features defined by a certain process of its manufacturing. The structure represents a grid having at least one cycle formed of at least two locally adjacent elements having different optical properties in respect of an Incident radiation. An optical model, based on at least some of the features of the structure is provided. The model is capable of determining theoretical data representative of photometric intensities of light components of different wavelengths specularly reflected from the structure and of calculating said at least one desired parameter of the structure. A measurement area, which is substantially larger than a surface area of the structure defined by the grid cycle, is illuminated by an incident radiation of a preset substantially wide wavelength range. Light component substantially specularly reflected from the measurement area is detected and measured data representative of photometric intensities of each wavelength within the wavelength range is obtained. The measured and theoretical data satisfies a predetermined condition. Upon detecting that the predetermined condition is satisfied, said at least one parameter of the structure is calculated.
The method disclosed in U.S. Pat. No. 6,281,974 suffers from basically the same drawbacks as U.S. Pat. No. 5,963,329—namely, that since only information in the zero order diffraction beam is recorded the profile determination is only sufficient for symmetric structures. An attempt to apply the method of U.S. Pat. No. 6,281,974 to asymmetric profiles will lead to un-determined equation systems that are incapable of calculating the profile in an unambiguous manner.
In view of the problems and disadvantages discussed above there is a need for an improved technique for quality control of microstructures that reduces the profile measurements uncertainties and avoids damaging of the microstructure to be tested. Furthermore, the requirements for high quality profile measurements are relaxed with the present invention, thus decreasing the complexity and cost for quality control Inspection of microstructures.
Thus, it is an object of the present invention to provide a non-destructive optical method and an apparatus for rapid, accurate, reproducible, unique and reliable determination of the surface topography of samples having semi-periodic structures deposited directly onto a nearly planar sample, or onto a film deposited on a nearly planar sample, or embedded within a film.
It is a further object of the present invention to provide a method for determination of the 3D profile of the semi-periodic structure based on measurements of light diffracted from the sample having the semi-periodic structure.